When this collection is non-empty and MailBee is asked to get the DNS servers from the config file, MailBee will append entries from this collection to the entries found in the config file.
This is useful for .NET Core, UWP, Xamarin where app.config is not available or you need to set the servers in runtime and it's not possible to auto-detect them. So, even if you don't have any app.config
at all, you can mimic having some entries there by populating DnsServers collection.